In automation technology, sensors are often used, which have a non-linear, measuring-transducer characteristic curve. In such case, the measurement signal of the measuring transducer depends non-linearly on the process variable to be measured, e.g. pressure or temperature, etc. In order to obtain a linear output signal of the sensor, e.g. for a 4-20 mA current loop, a processing of the measurement signal is necessary. This processing is done with the help of a linearizing curve, which normally is specified via a plurality of support points. Since, in practice, other points of the linearizing curve are needed, corresponding approximations are needed.
A very simple approximation method is linear interpolation. In such case, the support points are connected together via straight-line segments. This type of interpolation is, however, not sufficient for the accuracy requirements of modern sensors, above all, when the support points lie relatively far from one another. Better methods for obtaining the linearizing curve are polynomial approximation or spline approximation.
These approximation methods require, however, a very high calculational effort. Calculational capacities in the case of sensors of automation technology are, however, partially very limited, especially in the case of sensors which are supplied with energy via the communication connection, i.e. so-called 2-conductor devices. The firm Endress+Hauser produces and distributes a large number of such sensors.
A further disadvantage of these approximation methods is that the ascertained curve does not pass exactly through the predetermined support points. Thus, an assumption is that the linearizing curve is exact at the support points. Additionally, in the case of a polynomial approximation, so-called “over-oscillators” can arise, which lead to inaccuracies.